1. Technical Field
The technical field relates to auxiliary power units for vehicles and more particularly to auxiliary power units providing start cranking for vehicle primary engines.
2. Description of the Problem
As part of tighter motor vehicle emission standards, commercial operators of trucks will be required to avoid long term idling of diesel engines. Extended idling has been a common practice among truck drivers who park their rigs for mandatory break periods and desire to have power for heating, cooling and entertainment while on break. To meet the demand for power, heating and cooling during break periods, auxiliary power units (APU's) have long been used on some trucks for the fuel savings they produce against running larger displacement primary engines. Interest in APU's for trucks has greatly intensified in recent years due to motor vehicle emission standards and higher fuel prices. These factors are overcoming the disadvantages long associated with APU's such as bulkiness and added weight.
The revived interest in APU's has also revived the dilemma of how best to apply such systems. It is more or less a given that they should be used to supply the vehicle with an auxiliary source of electrical power. The art though reflects a debate about the degree to which the APU should be interconnected with a vehicle's main engine. There are a number of trade-offs to consider, for example, an APU based on an air-cooled internal combustion engine saves on vehicle weight, but if the auxiliary engine is air cooled, heated coolant is not available to warm the vehicle's coolant for circulation through a cold compression ignition engine to ease starting of that engine.
The degree to which an auxiliary engine is interconnected with the vehicle's primary engine reflects an underlying debate about the possible functions that an auxiliary engine might fulfill beyond providing electrical power and heat. Starter engines have long been employed with heavy duty compression ignition engines due to the difficulties associated with starting such engines in cold weather, although in the early art such starter engines were not described as fulfilling the functions commonly thought of for contemporary auxiliary power units. A starter engine requires coupling to the main engine to provide cranking of the engine. Typically some sort of mechanical or hydraulic transmission has been provided. U.S. Pat. No. 2,943,617 exemplifies a hydraulic drive for a starter motor. U.S. Pat. No. 3,991,734 illustrated mechanical coupling of an “auxiliary engine” to supplement an electrical starter motor or air starter. U.S. Pat. No. 5,528,901 proposes using an auxiliary engine to support pressurizing air for an air starter, allowing the conventional heavy duty battery pack and electric starter motor to be eliminated.